1. Field of the Invention
The present invention relates to a method of fabricating an ultrasonic probe in which inputted electric signals are converted into ultrasounds which are transmitted, while the received ultrasounds are converted into electric signals which are outputted.
2. Description of the Related Art
There has been generally used an ultrasonic diagnostic system for facilitating diagnoses of diseases of the viscera and the like, in which ultrasonic beams are transmitted inside of the human body and ultrasounds reflected by a tissue in the human body are received in the form of received signals, so that images in the human body involved in the received signals are displayed. In such an ultrasonic diagnostic system, there is used an ultrasonic probe which serves as a transducer wherein electric signals are converted into ultrasounds which are transmitted into the subject, while ultrasounds reflected by the inside of the subject are received and converted into electric signals.
FIG. 9 is a perspective view of an ultrasonic probe illustrated by way of example. FIG. 10 is a block diagram of circuits to be connected to the ultrasonic probe shown in FIG. 9.
The ultrasonic probe comprises a number of piezoelectric transducers 1 each made of for example piezo-electric ceramic (PZT ceramics) which are arranged in an array configuration in the horizontal direction (scan direction: x-direction). At the front side of an array of the piezoelectric transducers 1, there are formed common front electrodes 1a which are electrically connected with each other and earthed. And at the rear side of the array of the piezoelectric transducers 1, there are formed rear electrodes 1b which are independently of each other and are each connected to the associated lead wire 2. Further, at the front side of the array of the piezoelectric transducers 1, there are formed matching layers 3 each made of epoxy resin or the like and corresponding to the associated piezoelectric transducer 1, and in addition there is provided an acoustic lens 4 made of silicone rubber or the like for converging the ultrasounds transmitted from the ultrasonic probe with respect to the minor direction (y-direction) perpendicular to the scan direction (x-direction). And at the rear side of the array of the piezoelectric transducers 1, there is provided a backing 5 which is coupled through the rear electrode 1b to the array of the piezoelectric transducers 1, for the purpose of reducing a duration of waves of ultrasounds and also of absorbing the ultrasounds radiated toward the rear side.
To transmit the ultrasonic acoustic waves within the subject (not illustrated) such as the human body by the use of the ultrasonic probe having the arrangement as mentioned above, pulse signals are applied from a transmission circuit 6 shown in FIG. 10 to the piezoelectric transducers 1, respectively, so that the respective piezoelectric transducers 1 radiate burst waves of the ultrasonic beams. The pulse signals, which are transmitted from the transmission circuit 6, are controlled in timing of transmission of the pulse signals in such a manner that the ultrasonic beams radiated from the respective piezoelectric transducers 1 are converged on a predetermined depth position inside of the subject.
Meanwhile, the ultrasounds radiated from the ultrasonic probe and reflected by the inside of the subject are received by the respective piezoelectric transducers 1 and be converted into received signals. The received signals are amplified suitably and then applied to a beamformer circuit 8 in which beamforming operation is conducted in such a way that the ultrasonic beams radiated from the respective piezoelectric transducers 1 are converged on a fixed or sequentially varied depth position inside of the subject.
The received signals undergone the beamforming operation in the beamformer circuit 8 are inputted to a signal processing circuit (not illustrated) in which image signals representative of images inside of the subject by the ultrasounds are generated on the basis of the received signals. The images may be displayed on, for example, a CRT display and the like, in accordance with the thus obtained image signals.
FIG. 11A is a view showing a sound pressure distribution of ultrasounds radiated from an array of piezoelectric transducers, FIG. 11B is a view showing a sound pressure profile of a section of an ultrasonic beam in case of the sound pressure distribution of radiation as shown in FIG. 11A, and FIG. 11C is a graphical representation showing the relations between the depth within the subject and the beam width of the minor axis direction, in case of the sound pressure distribution of radiation as shown in FIG. 11A.
As shown in FIG. 11A, when ultrasonic acoustic waves, which have even sound pressure of radiation throughout the center portion and the both edge portions of the piezoelectric transducers with respect to the minor axis direction, are sent out, there will be induced large side lobes on the ultrasonic beams within the subject, as shown in FIG. 11B. As a result, as shown in FIG. 11C, it is merely permitted that the ultrasonic beams are contracted only at the neighborhood of the focal point of the acoustic lens (refer to FIG. 9), and thus the beam diameter will be remarkably spread.
FIGS. 12A-12C are views similar to FIGS. 11A-11C, respectively, except that the sound pressure distribution of radiation is different with respect to the minor axis direction.
As shown in FIG. 12A, there is prepared an array of piezoelectric transducers each having a long board-like configuration and being arranged in the scan direction. The piezoelectric transducer is polarized in such a manner that intensity of the polarization is stepwise given with respect to the minor axis direction. To polarize the piezoelectric transducer in such a way that intensity of the polarization is stepwise given with respect to the minor axis direction, there is used, for example, the following technique.
FIG. 14 is a typical illustration showing a technique of polarizing the piezoelectric transducer in such a manner that intensity of the polarization is stepwise given with respect to the minor axis direction.
Generally, according to the polarization operation for the piezoelectric transducer, a hundreds volts/mm of electric field is applied for several minutes to several hours to good conductor electrodes disposed at two main planes (a front face and a rear face) of a transducer plate, which are placed over against each other, in the tens to hundreds .degree.C. of temperature atmosphere. The polarization states can be controlled in accordance with these polarization conditions, that is, the polarization electric field, the polarization temperature and the polarization time. Setting up of a larger value of the polarization condition causes the polarization state of the piezoelectric transducer to be larger as a process proceeds from the unsaturation polarization to the saturation polarization.
For those reasons, as shown in FIG. 14, there are formed on one main plane (e.g. the rear face) of the piezoelectric transducer a plurality of good conductor electrodes each having a stripe-like configuration and extending in the longitudinal direction of the piezoelectric transducer, or the scan direction (a x-direction; refer to FIG. 9) when constructed as the ultrasonic probe, while a sheet of good conductor electrode is formed as a whole on another main plane (e.g. the front face) of the piezoelectric transducer. And between the respective stripe-like shaped electrodes at the rear face side and the opposite front side of electrode, as seen from the figure, applied are voltages which are larger as the location of the associated electrode is closer to the center of the piezoelectric transducer with respect to the minor axis direction, and in this condition the piezoelectric transducer is placed for a predetermined polarization time under a predetermined polarization temperature. Thus, there is built the piezoelectric transducer having a step-like shaped distribution of intensity of the polarization.
In this manner, when the piezoelectric transducer is polarized in such a way that a distribution of intensity of the polarization is stepwise given with respect to the minor axis direction, there will be obtained an electromechanical coupling factor having a distribution in compliance with such distribution of intensity of the polarization. Where the electromechanical coupling factor k is defined by the square root of the ratio of electric energy inputted to the piezoelectric transducer to mechanical energy with which the piezoelectric transducer vibrates when the electric energy is applied thereto, that is, as follows: EQU k=(output mechanical energy/input electric energy).sup.1/2 ( 1)
The piezoelectric transducer undergone the polarization operation in the manner as mentioned above is segmented, as shown in FIG. 13B, into a number of pieces which are disposed on the ultrasonic probe to radiate ultrasounds. The radiated ultrasounds have each a sound pressure which is substantially in proportion to the electromechanical coupling factor. Consequently, in order to attain a desired distribution of sound pressure of radiation, it is sufficient to provide a desired distribution of electromechanical coupling factor. Since the electromechanical coupling factor depends on intensity of the polarization, control of intensity of the polarization of an piezoelectric transducer causes a desired distribution of sound pressure of radiation, or a desired weighting in amplitude of ultrasounds radiated from the piezoelectric transducer, to be available.
If it is desired to attain a distribution of intensity of the polarization as shown in FIG. 13A, it is sufficient to conduct the polarization up to a saturation state (saturation polarization) for a center of the piezoelectric transducer with respect to the minor axis direction, but it is necessary to conduct an unsaturation polarization for the edge portions thereof.
With respect to the polarization for a center of the piezoelectric transducer with respect to the minor axis direction, that is, the saturation polarization, it is possible to attain the saturation polarization by means of setting the polarization electric field, polarization temperature and/or polarization time as the aforementioned polarization conditions to be sufficiently large, thereby attaining a stabilized constant electromechanical coupling factor, since the polarization is saturated. However, regarding the polarization for the edge portions of the piezoelectric transducer with respect to the minor axis direction, that is, the unsaturation polarization, even though the polarization conditions are exactly controlled, it is difficult to attain a predetermined electromechanical coupling factor because of the peculiar polarization conditions.
FIG. 15 is a graphical representation showing the relations between the polarization condition and the electromechanical coupling factor by way of example.
The graph shown in FIG. 15 represents the relations between the polarization condition and the electromechanical coupling factor in a case where there are prepared a number of piezoelectric transducers, the polarization electric field E is fixed as E=350 V/mm, and the polarization is conducted while varying the polarization temperature and the polarization time.
As seen from FIG. 15, even though the polarization is implemented in the completely same condition, it happens that the electromechanical coupling factor of the piezoelectric transducer after polarization involves the amount of scatter not less than 10%. On the contrary, in order to implement a desired distribution of sound pressure as the aforementioned distribution of sound pressure, it is necessary to suppress the amount of scatter approximately less than 7%.
To suppress the amount of scatter, there is proposed a method (Japanese patent Laid Open Gazette No. 237351/1987) in which the polarization operation is implemented while the polarization state of the piezoelectric transducer is monitored. As disclosed in the above-noted Japanese patent Laid Open Gazette No. 237351/1987, it is possible to identify the polarization state of the piezoelectric transducer by means of measuring frequency characteristics of an electric impedance of the resonance neighborhood of the piezoelectric transducer, alternatively measuring amplitude characteristics of ultrasonic acoustic signals generated by the piezoelectric transducer. To say from the view point that the measurement is conveniently carried out, it is noted that the former, that is, the scheme of measuring frequency characteristics of an electric impedance is more practical.
If this scheme can be utilized well, the amount of scatter in the electromechanical coupling factor ought to be significantly decreased in accordance with such a way that the polarization operation is advanced while the polarization state is monitored, and the polarization operation is terminated when the piezoelectric transducer reaches in intensity of the polarization (electromechanical coupling factor) a predetermined value.
Generally, however, the piezoelectric transducer used in fabrication of the probe is provided with a relatively larger capacitance of the order of tens nF. This involves such a problem that it is difficult to exactly measure frequency characteristics of an electric impedance of the resonance neighborhood of the piezoelectric transducer. The reason why such frequency characteristics can not be measured is that the relatively larger capacitance of the order of tens nF causes a resonance in cooperation with a small inductor component (tens nF) existing on a cable used in measurement of an electric impedance, which cable is connected to the piezoelectric transducer, and thus the electric impedance can not be exactly measured. Even in case of the fabrication of one in which very small pieces of piezoelectric transducer are arranged, such as an array type of probe in which a number of piezoelectric transducers 1 are arranged, as shown in FIG. 9, the probe is temporarily assembled, usually, as mentioned in connection with the explanation of FIG. 13, in a state of piezoelectric transducer having a board-like configuration, and thereafter segmented into very small pieces. Hence, also in this case, when the polarization is carried out, there is given a state of board-like shaped piezoelectric transducer and thus the piezoelectric transducer is provided with a relatively larger capacitance. Accordingly, the similar inconvenience will occur.
As another problem, in case of the implementation of a piezoelectric transducer such that a plurality of stripe configuration of electrodes are arranged on the piezoelectric transducer as shown in FIG. 14, and the respective electrodes are supplied with mutually different polarization conditions (e.g. polarization electric field) so as to practice the above-mentioned amplitude weighting, when the polarization state under some stripe electrode is observed, as shown in FIG. 16, there is such a problem that the portions under other stripe electrodes serve as mechanical loads, and thus it is difficult to exactly measure the polarization state under the portion of interest.